[Amps] Plate choke magic?

[Tomm Aldridge]

Thanks Will, that makes perfect and practical sense.  Plate choke valuse 
for 1.8 through 30MHz amplifiers need to be high enough to present a 
large reactance at 1.8MHz with large defined as >> greater than the 
plate impedance, correct?  And they must not produce any resonances 
withing the 1.8 to 30MHz band, correct?  But, what about the fact that 
the tubes have gain well above 30MHz and well below it as well?

If I say that the plate Z is 2k ohms and therefore I want 20k ohms at 
1.8MHz to satisfy the >> larger condition above, I get an inductance of 
1.77mH.  Looking at some plate chokes for QRO amps out there, I see 
values in the range of 200uH (a bit greater than the plate Z) to 500uH, 
much lower than I would consider to be an effective choke.  A 300uH 
choke is approx 1" x 6" with 278 turns of 26AWG.  Seems to be a 
reasonable DCR to be putting in a plate circuit.  I calculate about 2.9 
DC ohms with a large surface to distribute the losses.  But why such a 
small inductance???

Tomm

Will Matney wrote:
> Tom,
> 
> Actually, a ferrite core can be used if it's of the correct type of  
> material. The material is determined by the frequency that the coil 
> will  operate at. There a couple of ferrite and iron powder types that 
> would  work. The reason most are air coils I would think is they are 
> cheaper to  make. An insulated form is all that's really needed. The air 
> coil formula  is then used to determine the number of turns for the 
> amount of inductance  wanted. The higher the frequency, the lesser 
> amount of inductance is  needed to block the RF, so the choke needs to 
> be designed around the  lesser frequency that will be encountered. Then 
> you need to make sure the  choke is not self-resonant at any frequency 
> you wish to operate it on.  This is done by using a grid dip meter and 
> shorting the coils leads  together. Any dip at any desired frequency 
> means that the inductance will  have to be changed slightly to move the 
> resonance point to where it wont  be encountered. Most of the time this 
> is done by simply adding or  shortening a few turns of wire. Those 
> staggered windings on some chokes  are done to stop self-resonance at a 
> particular operating frequency, and  are really several inductors being 
> connected in series where Ltotal = L1  + L2 + L3, etc.. Hope this helps 
> as an explanation.
> 
> Will
> 
> 
> 
> On Fri, 28 Jan 2005 23:42:10 +0000, Tomm Aldridge <KD7QAE at ARRL.NET> wrote:
> 
>> Why are plate chokes seemingly black magic?  Don't you just want a 
>> good  decoupling of the PS from the Plate; i.e. lots of impedance from 
>> DC to  Light and no resonances?  How I get that should not be an issue 
>> but all  teh plate chokes I see are long skinny and sometimes 
>> segmented single  layer solenoids of questionable wire size.  Why 
>> wouldn't a really lossy  powdered metal toroid with a few fat turns on 
>> it work, assuming the  inductance was high enough?
>>
>> KD7QAE
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